<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Transform + fixed</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }
    html, body {
      height: 100%;
      width: 100%;
    }
    body {
      display: flex;
      flex-direction: column;
    }
    .content {
      transform: scale(1);
      flex: 0 0 600px;
      border: 1px solid red;
    }
    .input {
      position: fixed;
      bottom: 0;
      left: 0;
      width: 100%;
      height: 100px;
      border: 1px solid blue;
      box-sizing: border-box;
    }
  </style>
</head>
<body>
  <div class="content">
    这是主题内容
    <div class="input fixed">
      这是一个输入框
    </div>
  </div>
</body>
</html>